This is the third in a series of photographs of Silurian rocks from Clogher Bay. A brief examination of the literature indicates that the rocks in these pictures belong to the Drom Point Formation which has accumulated to a depth of 300 metres and is part of the Dunquin Group of Silurian Period strata in Ireland. The Drom Point and Croagh-marhin Formations consist of shallow-marine, fossiliferous siltstones and very fine to fine grained sandstones.
You don’t have to be a rockhound to be impressed by the spectacular scenery at The Hopewell Rocks. Tall cliffs of sloping red strata rise high above the Bay of Fundy shore, with an abundance of naturally worked shapes, caves, arches, and free-standing pillars of rock called sea stacks. At high tide, people can kayak around the stacks, also known locally as “Flower Pots” because of the groups of full-grown trees that grow on top of them – as they also do right to the cliff edges, with their root systems often clearly visible. At low tide, it is possible to descend a staircase to the ocean floor itself and explore these geological phenomena close up. Viewing time on the seashore is limited by the enormous and potentially dangerous rise and fall of the tides in this narrower northern neck of the Bay, where in some places, and at certain times, the sea can rise by as much as 56 feet.
At one time, about 600 million years ago, this part of Canada’s New Brunswick Province started its life near the Equator. Here it was subjected to uplifting earth movements that incorporated it into the Appalachian Oregon, an ancient mountain chain that now stretches from New Foundland to Florida. By 360 million years ago, the Appalachian building activities had ended and were followed by predominantly erosional processes.
The rocks exposed at Hopewell originated specifically in that part of the Appalachians called the Caledonian Mountains. Erosion by water and wind about 350 million years ago, in the Lower Carboniferous Period, steadily wore down the mountains, creating massive volumes of boulders, stones, gravel, sand and mud. Near the highland areas, flash floods tore through the valleys and canyons, washing away loads of eroded sediment and depositing it as stony and gravelly debris. Further from the highlands, sediment formed alluvial plains with sorted layers of sand and mud. The region covered by these terrestrial deposits in present day Atlantic Canada is called the Maritime Basin.
Over time, the coarser material in the erosion deposits on the flood plain became consolidated and cemented together with finer sand and silt. Because the land lay near the equator, the climate was hot and dry. Iron-bearing minerals became oxidised, and the rocks turned into redbeds. The series of red rock layers is now known as the Hopewell Cape Formation; this is the rock exposed in the cliffs and sea stacks at Hopewell today – eventually brought to its current position by Continental Drift, the tectonic movement of continental crustal plates.
In the first instance, the variably-textured sedimentary strata were deposited in horizontal layers. However, earth movements tilted them to angles between 30 and 45 degrees. The tilting of the rocks caused horizontal cracks to form parallel to the bedding planes, and also vertically at right angles to the strata. These lines of weakness in the rocks have become the points of entry for weathering agents – glaciers, tides, snow, ice, and winds. Erosion by these forces widens the cracks and steadily works away at the softer horizontal strata. The expansion of water as it changes to ice is a significant factor in the enlargement of cracks and crevices, and the breaking up the rock. Sandstone is softer than the conglomerate and easy for waves to wear away. The overall result is that broad columns of rock are carved into the cliff face. Undercutting at the cliff base creates caves and arches. Eventually, some columns are completely separated from the cliff face and become sea-stacks or “flower pots”.
The erosion activities are on-going. Extreme weather events and storms of recent years may accelerate the processes. The cliff face is gradually receding. Sea stacks eventually collapse and new ones are formed. A sea stack can last as little as 100 years or as long as a thousand. However, there is no need to panic about seeing the sights at Hopewell as soon as possible for fear that they will all disappear into the sea – geologists have calculated that there is enough conglomerate in the Hopewell Cape Formation to make “flower pots” for the next 100,000 years.
Chondrites are trace fossils or ichnofossils. They are small branching burrows or tunnels that were made while the sediments were still soft and have subsequently become preserved in the hardened strata. There is a great deal of uncertainty about which organisms created the burrows because no animal has ever been found within them – but they may have been some kind of small marine worm. There is evidence to support the idea that the burrows were formed in sediments with reduced oxygen or none at all.
The trace fossil Chondrites, a highly branched burrow system of unknown endobenthic deposit feeders, occurs in all types of sediment, including those deposited under anaerobic conditions. In some cases, such as the Jurassic Posidonienschiefer Formation of Germany, Chondrites occurs in black, laminated, carbonaceous sediment that was deposited in chemically reducing conditions. In other cases, such as numerous oxic clastic and carbonate units throughout the geologic column, Chondrites typically represents the last trace fossil in a biotutbation sequence. This indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters.
The Chondrites shown in these pictures occurred in Silurian rocks of the Dunquin Group on the Dingle Peninsula in western Ireland. Some were found in beach stones at the northern end of Smerwick Harbour, however, the majority were photographed in Clogher Bay on large boulders and in bedrock.
Uncannily realistic, this life-sized and life-like model figure featured in an exhibition about early humans at the Natural History Museum in London. It was called “One million years of the human story“. The details of the reconstruction are based on information gained from burial remains recovered from Paviland in Wales, dating from 30,000 years ago.
Homo sapiens, our species, evolved in Africa and started to spread into other parts of the world around 60,000 years ago, arriving in Britain at least 40,000 years ago. We probably evolved from an earlier human species, Homo heidelbergensis.
In an earlier post about rock textures and patterns at Tenby in South Wales I said that some rock surfaces reminded me of elephant hide. So, shown above are a few photographs that I took of the elephant skin on a prepared specimen exhibited at the Natural History Museum in London to show you what I meant – while below are a couple of examples of the textured limestone from Tenby for comparison.